<!DOCTYPE html>
<style>
.container > * {
    position: absolute;
}
.large {
    width: 50px;
    height: 50px;
}
.small {
    left: 25px;
    height: 25px;
    width: 50px;
}
</style>
<div>This tests that z-index on non-positioned flex-items works. The green boxes should be above the orange boxes, which should be above the purple boxes, which are above the salmon boxes.<div>
<div class="container" style="position:relative">
    <div class="large" style="z-index: 1; background-color: salmon;"></div>
    <div class="large" style="z-index: 100; background-color: orange; left: 50px;"></div>
    <div class="small" style="z-index: 150; background-color: green"></div>
    <div class="small" style="z-index: 50; background-color: purple; top: 25px;"></div>
</div>
<div class="container" style="position:relative; top: 50px">
    <img class="large" style="z-index: 1; background-color: salmon;"></img>
    <img class="large" style="z-index: 100; background-color: orange; left: 50px;"></img>
    <img class="small" style="z-index: 150; background-color: green"></img>
    <img class="small" style="z-index: 50; background-color: purple; top: 25px;"></img>
</div>
